AP_NavEKF3: fixed null dereference of airspeed

this caused a segfault in replay

libraries/AP_NavEKF3/AP_NavEKF3_Outputs.cpp

@@ -46,7 +46,7 @@ float NavEKF3_core::errorScore() const
         // EKF less sensitive to innovations arising due events like strong gusts of wind, thus, prevent reporting high error scores
         if (assume_zero_sideslip()) {
             const auto *arsp = dal.airspeed();
-            if (arsp->get_num_sensors() >= 2 && (frontend->_affinity & EKF_AFFINITY_ARSP)) {
+            if (arsp != nullptr && arsp->get_num_sensors() >= 2 && (frontend->_affinity & EKF_AFFINITY_ARSP)) {
                 score = MAX(score, 0.3f * tasTestRatio);
             }
         }